lc.scm: Use getopt-long.  Update help messages.

* ui/lc.scm (lazycat-add, lazycat-diff): Use getopt-long for
  parsing arguments.  Update the help message.
  (lazycat-rem, lazycat-exec, lazycat-set, lazycat-get)
  (lazycat-list, main): Update the help message.